Path: utzoo!utgpu!news-server.csri.toronto.edu!torsqnt!hybrid!robohack!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Newsgroups: comp.graphics
Subject: Re: RGB-=>CMYK
Message-ID: <1990Jun3.231332.1551@druid.uucp>
Date: 3 Jun 90 23:13:32 GMT
References: <2967@crash.cts.com>
Reply-To: darcy@druid.UUCP (D'Arcy J.M. Cain)
Organization: D'Arcy Cain Consulting, West Hill, Ontario
Lines: 29

In article <2967@crash.cts.com> jandreas@pro-graphics.cts.com
(Jason Andreas) writes:
>I'm in need of some software (specifically source code) for RGB-=>CMYK
>conversion.  If someone could post a phone number or FTP site where such a
>thing is available, please do so.
>
Perhaps this should be added to the FAQ.

This depends on how you store colour information but assuming each of
the RGB and CMYK components is an integer from 0 to 255 then you have
to do the following:

Convert to CMY
    C = 255 - R
    M = 255 - G
    Y = 255 - B

Find the black level.  This is simply the smallest of C, M or Y.  Subtract
this value from each of C, M and Y.  Note that this means that even with
black included, there is never more than 3 components for a colour.

There are further enhancements which involve adjusting for each of the inks
but this varies with different printing processes.

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